1. Technical Field
The present invention relates in general to data transmission and in particular to extending the distance over which data may be transmitted on a transmission line which inherently distorts the transmission. Still more particularly, the present invention relates to introducing predistortion to a data transmission waveform which is the inverse of distortion inherent in a transmission line.
2. Description of the Related Art
Digital data is typically transmitted over various metallic transmission media, including coaxial cables, in the form of a series of square waves or pulses. Digital signals transmitted over cables or other transmission media may be severely distorted due to attenuation losses inherent in the cables. Such cable losses limit the data transmission rate and/or the distance over which signals may be transmitted.
The data transmission rate for an information bearing signal on a transmission line is limited by data jitter, or the tendency of transitions to occur at different points during the respective signal periods. Data jitter arises as a result of two phenomenon: transmission line attenuation typically increases with frequency; and data transitions in adjacent signal periods appear, to the transmission line, as a signal with a different set of frequencies than transitions which are separated by several signal periods. For example, transmission of the pattern 10101010 will be attenuated differently than transmission of 10011001. The higher attenuation causes signals with irregular transitions to be more severely distorted, with transitions occurring at different points during the signal period than in signals with regular transitions. This limits the pulse width, and thus the data rate, at which data may be accurately transmitted and received.
The distance over which an information bearing signal may be transmitted on a transmission line is primarily limited by the phase distortion introduced by the transmission line. Progressive alteration of the pulse shape from the predetermined pulse shape for which a receiver is designed to operate may lead to transmission errors and limit the maximum range of the transmission.
Several prior art approaches attempt to alleviate the problems arising from distortion. One approach, known as receiver equalization, involves processing the received signal to make it more nearly correspond to the predetermined pulse shape. However, receiver equalization is often complicated by overlap in adjacent pulses in high speed transmission systems.
A second prior art approach, referred to as predistortion or transmitter equalization, employs a mechanism at the output of the transmitter to alter the shape of the pulse introduced onto the transmission line. The predistortion is intended to be the inverse of the distortion resulting from inherent cable losses. This technique is suitable when an estimate of the distortion introduced by the transmission line is available, as where the length and material of the transmission line is known. A variant of this approach, known as adaptive predistortion, attempts to accommodate differing lengths in the transmission lines. However, in both the basic technique and the variant of adaptive predistortion, a source of clock pulses is typically required to operate the predistortion mechanism. Moreover, where digital filter techniques are used to synthesize a specific pulse shape approximating the ideal predistorted waveform, the required apparatus is very complex and occupies a significant amount of chip area in an integrated circuit. Another predistortion technique applies voltage to the output signal based on whether adjacent data signals are identical or different. Because cable distortion affects all bits in the data stream and is not limited to specific bits, this technique is limited in how closely the predistortion can be made to match the inverse of the cable distortion.
Transmitter or receiver equalizers are the most commonly used technique for dealing with cable attenuation. Passive external components are used to form a filter circuit which has a response approximating the inverse of the cable response, with the "gain" of the filter set to equal the loss of the cable.
Another prior art approach to countering transmission line distortion, which may be used in conjunction with receiver or transmitter equalization, involves special encoding of the data to be transmitted. However, predistortion techniques for such signals may be unsuitable for transmitting unencoded data, also known as NRZ data.
It would be desirable to introduce predistortion to a data transmission waveform which is the inverse of distortion inherent in a transmission line in a simple manner. It would be advantageous if the mechanism for introducing such predistortion did not require a clock pulse or special encoding of the data to be transmitted. It would also be desirable for the mechanism to support transmission of DC coupled data and to introduce varying amounts of predistortion to all bits of the data transmitted.